Laser welding and laser-based powder-bed fusion additive manufacturing in the deep penetration(keyhole)mode are promising technologies for the synthesis of metal components.The significant potential of these technolog...Laser welding and laser-based powder-bed fusion additive manufacturing in the deep penetration(keyhole)mode are promising technologies for the synthesis of metal components.The significant potential of these technologies remains latent because of structural defects(porosity),which significantly degrade the structural integrity and performance of the end products.Practical strategies for reducing those defects are addressed through fundamental understanding of their formation.In this study,pore formation of hydrodynamic origin is investigated,including the dynamics and mechanisms of the formation based on the above mentioned technologies.The pore volume and frequency of pore appearance,depending on the amplitude and frequency of capillary vibrations,are considered.Physical analysis is performed to obtain the scanning velocity values for the maximum and zero amplitudes and the frequency of capillary waves.A comparison between calculated curves and experimental data confirms both the capillary origin of the pores and the estimated scanning speeds at which the parameters of the pores exhibit their maximum values or vanish.The results obtained may facilitate in the selection of the optimal scanning speed when designing a pore-free technology.展开更多
The surface of nodular cast iron （NCI） with a ferrite substrate was rapidly remelted and solidified by plasma transferred arc （PTA） to induce a chilled structure with high hardness and favorable wear resistance. T...The surface of nodular cast iron （NCI） with a ferrite substrate was rapidly remelted and solidified by plasma transferred arc （PTA） to induce a chilled structure with high hardness and favorable wear resistance. The effect of scanning speed on the microstructure, micro-hardness distribution, and wear properties of PTA-remelted specimens was systematically investigated. Microstructural characterization in-dicated that the PTA remelting treatment could dissolve most graphite nodules and that the crystallized primary austenite dendrites were transformed into cementite, martensite, an interdendritic network of ledeburite eutectic, and certain residual austenite during rapid solidifica-tion. The dimensions of the remelted zone and its dendrites increase with decreased scanning speed. The microhardness of the remelted zone varied in the range of 650 HV0.2 to 820 HV0.2, which is approximately 2.3-3.1 times higher than the hardness of the substrate. The wear re-sistance of NCI was also significantly improved after the PTA remelting treatment.展开更多
Scanning speed is a critical parameter for laser process, which can play a key role in the microstruc- ture evolution of laser melting. In the laser melting of single crystal superalloy, the effects of scanning speed ...Scanning speed is a critical parameter for laser process, which can play a key role in the microstruc- ture evolution of laser melting. In the laser melting of single crystal superalloy, the effects of scanning speed were investigated by experimental analysis and computational simulation. The laser was scanning along [710] direction on （001） surface in different speeds. Solidification microstructures of dendrites growth direction and the primary dendritic spacing were analyzed by metallograph. Besides, a planar interface during solidification was taken into attention, Experiment results indicated that the primary dendritic spacing and thickness of planar interface decrease with the increase of speed. Through simu- lation, distribution of dendrites growth velocity and thermal gradient along dendrite growth direction were calculated, and the simulation of dendrites growth direction agreed with the experiment results. Additionally, a constant value was acquired which can be used to predict the primary dendritic spacing. Moreover, according to curve-fitting method and inequality relation, a model was proposed to predict the thickness of planar interface.展开更多
A scanning microwave radiometer（RM） was launched on August 16,2011,on board HY-2 satellite.The six-month long global sea surface wind speeds observed by the HY-2 scanning microwave radiometer are preliminarily valid...A scanning microwave radiometer（RM） was launched on August 16,2011,on board HY-2 satellite.The six-month long global sea surface wind speeds observed by the HY-2 scanning microwave radiometer are preliminarily validated using in-situ measurements and WindSat observations,respectively,from January to June 2012.The wind speed root-mean-square（RMS） difference of the comparisons with in-situ data is 1.89 m/s for the measurements of NDBC and 1.72 m/s for the recent four-month data measured by PY30-1 oil platform,respectively.On a global scale,the wind speeds of HY-2 RM are compared with the sea surface wind speeds derived from WindSat,the RMS difference of 1.85 m/s for HY-2 RM collocated observations data set is calculated in the same period as above.With analyzing the global map of a mean difference between HY-2 RM and WindSat,it appears that the bias of the sea surface wind speed is obviously higher in the inshore regions.In the open sea,there is a relatively higher positive bias in the mid-latitude regions due to the overestimation of wind speed observations,while the wind speeds are underestimated in the Southern Ocean by HY-2 RM relative to WindSat observations.展开更多
The influence of rare earth （RE） elements on the solidification process and eutectic transformation and mechanical properties of the high-V type cast, high-speed steel roll was studied. Test materials with different...The influence of rare earth （RE） elements on the solidification process and eutectic transformation and mechanical properties of the high-V type cast, high-speed steel roll was studied. Test materials with different RE additions were prepared on a horizontal centrifugal casting machine. The solidification process, eutectic structure transformation, carbide morphology, and the elements present, were all investigated by means of differential scanning calorimetry （DSC） and scanning electron microscopy energy dispersive spectrometry （SEM-EDS）. The energy produced by crack initiation and crack extension was analyzed using a digital impact test machine. It was found that rare earth elements increased the tensile strength of the steel by inducing crystallization of earlier eutectic γ-Fe during the solidification process, which in turn increased the solidification temperature and thinned the dendritic grains. Rare earth elements with large atomic radius changed the lattice parameters of the MC carbide by forming rare earth carbides. This had the effect of dispersing longpole M C carbides to provide carbide grains, thereby, reducing the formation of the gross carbide and making more V available, to increase the secondary hardening process and improve the hardness level. The presence of rare earth elements in the steel raised the impact toughness by changing the mechanism of MC carbide formation, thereby increasing the crack initiation energy.展开更多
The wind effects on steady-state scan characteristics and hit probability of terminal-sensitive projectile were discussed in this paper. Considering wind as the constitutions of the average wind and the impulsive wind...The wind effects on steady-state scan characteristics and hit probability of terminal-sensitive projectile were discussed in this paper. Considering wind as the constitutions of the average wind and the impulsive wind, a simplified wind field model was established for the ballistic calculation of the steady-state scan phase; under the windy condition, the effects of the range wind and the beam wind on the steady-state scan characteristics of the terminal-sensitive projectile were analyzed in detail and its hit probabilities for a certain armored target were calculated. The calculated results show that, when the wind speed exceeds a certain value, the hit probabilities of terminal-sensitive projectile drop rapidly; the wind effects must be considered in the application of the terminal-sensitive projectiles. This paper provides some theoretical references for the fire wind speed correction and the global structure optimization of the terminal-sensitive projectile.展开更多
通过改变激光粉末床熔融(laser powder bed fusion,LPBF)的扫描速度研究IN738LC合金组织演化及各向异性机制,采用光学显微镜(optical microscopy,OM)及扫描电镜(scanning electron microscopy,SEM)对组织形貌特征进行表征分析,通过X射...通过改变激光粉末床熔融(laser powder bed fusion,LPBF)的扫描速度研究IN738LC合金组织演化及各向异性机制,采用光学显微镜(optical microscopy,OM)及扫描电镜(scanning electron microscopy,SEM)对组织形貌特征进行表征分析,通过X射线衍射(X-ray diffraction,XRD)对其织构性进行测试,使用显微硬度仪对显微硬度及各向异性进行评价.结果表明,随着扫描速度从800 mm/s提高到1600 mm/s,晶粒尺寸得到显著细化,且晶粒长轴取向由低扫描速度下沿建造方向择优,转变为高扫描速度下的沿熔池边界法线方向择优.这是因为低扫描速度下高熔池重熔率导致更多枝晶沿建造方向外延择优生长.这种沿建造方向的强择优生长同时导致(200)面沿建造方向择优的织构性,且这种织构强度随扫描速度增加而降低.这种(200)面沿建造方向择优织构还导致水平截面软轴居多,进而导致水平显微硬度低于侧界面显微硬度的各向异性.展开更多
基金Ministry of Science and Higher Education within the State Assignment to Federal Research Centre≪Crystallography and Photonics≫Russian Academy of Science.
文摘Laser welding and laser-based powder-bed fusion additive manufacturing in the deep penetration(keyhole)mode are promising technologies for the synthesis of metal components.The significant potential of these technologies remains latent because of structural defects(porosity),which significantly degrade the structural integrity and performance of the end products.Practical strategies for reducing those defects are addressed through fundamental understanding of their formation.In this study,pore formation of hydrodynamic origin is investigated,including the dynamics and mechanisms of the formation based on the above mentioned technologies.The pore volume and frequency of pore appearance,depending on the amplitude and frequency of capillary vibrations,are considered.Physical analysis is performed to obtain the scanning velocity values for the maximum and zero amplitudes and the frequency of capillary waves.A comparison between calculated curves and experimental data confirms both the capillary origin of the pores and the estimated scanning speeds at which the parameters of the pores exhibit their maximum values or vanish.The results obtained may facilitate in the selection of the optimal scanning speed when designing a pore-free technology.
基金financially supported by the Key Project of China National Erzhong Group Co.(No.2012zx04010-081)
文摘The surface of nodular cast iron （NCI） with a ferrite substrate was rapidly remelted and solidified by plasma transferred arc （PTA） to induce a chilled structure with high hardness and favorable wear resistance. The effect of scanning speed on the microstructure, micro-hardness distribution, and wear properties of PTA-remelted specimens was systematically investigated. Microstructural characterization in-dicated that the PTA remelting treatment could dissolve most graphite nodules and that the crystallized primary austenite dendrites were transformed into cementite, martensite, an interdendritic network of ledeburite eutectic, and certain residual austenite during rapid solidifica-tion. The dimensions of the remelted zone and its dendrites increase with decreased scanning speed. The microhardness of the remelted zone varied in the range of 650 HV0.2 to 820 HV0.2, which is approximately 2.3-3.1 times higher than the hardness of the substrate. The wear re-sistance of NCI was also significantly improved after the PTA remelting treatment.
基金financially supported by the National Natural Science Foundation of China (NSFC) under grant Nos.51401210 and 51271186
文摘Scanning speed is a critical parameter for laser process, which can play a key role in the microstruc- ture evolution of laser melting. In the laser melting of single crystal superalloy, the effects of scanning speed were investigated by experimental analysis and computational simulation. The laser was scanning along [710] direction on （001） surface in different speeds. Solidification microstructures of dendrites growth direction and the primary dendritic spacing were analyzed by metallograph. Besides, a planar interface during solidification was taken into attention, Experiment results indicated that the primary dendritic spacing and thickness of planar interface decrease with the increase of speed. Through simu- lation, distribution of dendrites growth velocity and thermal gradient along dendrite growth direction were calculated, and the simulation of dendrites growth direction agreed with the experiment results. Additionally, a constant value was acquired which can be used to predict the primary dendritic spacing. Moreover, according to curve-fitting method and inequality relation, a model was proposed to predict the thickness of planar interface.
基金The National High-Tech Project of China under contract No.2008AA09A403the Marine Public Welfare Project of China under contract No.201105032
文摘A scanning microwave radiometer（RM） was launched on August 16,2011,on board HY-2 satellite.The six-month long global sea surface wind speeds observed by the HY-2 scanning microwave radiometer are preliminarily validated using in-situ measurements and WindSat observations,respectively,from January to June 2012.The wind speed root-mean-square（RMS） difference of the comparisons with in-situ data is 1.89 m/s for the measurements of NDBC and 1.72 m/s for the recent four-month data measured by PY30-1 oil platform,respectively.On a global scale,the wind speeds of HY-2 RM are compared with the sea surface wind speeds derived from WindSat,the RMS difference of 1.85 m/s for HY-2 RM collocated observations data set is calculated in the same period as above.With analyzing the global map of a mean difference between HY-2 RM and WindSat,it appears that the bias of the sea surface wind speed is obviously higher in the inshore regions.In the open sea,there is a relatively higher positive bias in the mid-latitude regions due to the overestimation of wind speed observations,while the wind speeds are underestimated in the Southern Ocean by HY-2 RM relative to WindSat observations.
基金Project supported by"863"Project (2006AA03Z532)the National Natural Science Foundation of China (NSFC 50341050)
文摘The influence of rare earth （RE） elements on the solidification process and eutectic transformation and mechanical properties of the high-V type cast, high-speed steel roll was studied. Test materials with different RE additions were prepared on a horizontal centrifugal casting machine. The solidification process, eutectic structure transformation, carbide morphology, and the elements present, were all investigated by means of differential scanning calorimetry （DSC） and scanning electron microscopy energy dispersive spectrometry （SEM-EDS）. The energy produced by crack initiation and crack extension was analyzed using a digital impact test machine. It was found that rare earth elements increased the tensile strength of the steel by inducing crystallization of earlier eutectic γ-Fe during the solidification process, which in turn increased the solidification temperature and thinned the dendritic grains. Rare earth elements with large atomic radius changed the lattice parameters of the MC carbide by forming rare earth carbides. This had the effect of dispersing longpole M C carbides to provide carbide grains, thereby, reducing the formation of the gross carbide and making more V available, to increase the secondary hardening process and improve the hardness level. The presence of rare earth elements in the steel raised the impact toughness by changing the mechanism of MC carbide formation, thereby increasing the crack initiation energy.
基金Sponsored by Doctoral Foundation of Ministry of Education of China (20093219120006)
文摘The wind effects on steady-state scan characteristics and hit probability of terminal-sensitive projectile were discussed in this paper. Considering wind as the constitutions of the average wind and the impulsive wind, a simplified wind field model was established for the ballistic calculation of the steady-state scan phase; under the windy condition, the effects of the range wind and the beam wind on the steady-state scan characteristics of the terminal-sensitive projectile were analyzed in detail and its hit probabilities for a certain armored target were calculated. The calculated results show that, when the wind speed exceeds a certain value, the hit probabilities of terminal-sensitive projectile drop rapidly; the wind effects must be considered in the application of the terminal-sensitive projectiles. This paper provides some theoretical references for the fire wind speed correction and the global structure optimization of the terminal-sensitive projectile.
文摘通过改变激光粉末床熔融(laser powder bed fusion,LPBF)的扫描速度研究IN738LC合金组织演化及各向异性机制,采用光学显微镜(optical microscopy,OM)及扫描电镜(scanning electron microscopy,SEM)对组织形貌特征进行表征分析,通过X射线衍射(X-ray diffraction,XRD)对其织构性进行测试,使用显微硬度仪对显微硬度及各向异性进行评价.结果表明,随着扫描速度从800 mm/s提高到1600 mm/s,晶粒尺寸得到显著细化,且晶粒长轴取向由低扫描速度下沿建造方向择优,转变为高扫描速度下的沿熔池边界法线方向择优.这是因为低扫描速度下高熔池重熔率导致更多枝晶沿建造方向外延择优生长.这种沿建造方向的强择优生长同时导致(200)面沿建造方向择优的织构性,且这种织构强度随扫描速度增加而降低.这种(200)面沿建造方向择优织构还导致水平截面软轴居多,进而导致水平显微硬度低于侧界面显微硬度的各向异性.
